Barcode readers or scanners are found in many commercial environments such as, for example, point-of-sale (POS) stations in retail stores and supermarkets, inventory and document tracking, and diverse data control applications. To meet the growing demands, barcode symbol readers of various types have been developed for scanning and decoding barcode symbol patterns and producing symbol character data for use as input in automated data processing systems. Barcode scanners generally are available in hand-held, hands-free or in-counter formats.
Conventional laser barcode scanners generally include a laser generator such as a solid state visible laser diode (VLD) that generates a visible laser beam used for scanning and reading a barcode. The laser beam is directed at the target barcode through a laser output or exit lens or window made of an optically transparent material or medium such as plastic or glass. In some embodiments, the output windows may be tinted a color such as red as commonly used. The laser output windows are generally made by a molding process. The laser beam, which may be emitted by a laser diode housed within the scanner, is typically scanned or oscillated rapidly back and forth across the output window by some conventional means known in the art, such as a flipper as shown in FIG. 4. These scanners, referred to as “flying spot” laser scanners, moves or scans the laser beam leaving the output window across the entire barcode reflecting light back to a photodiode in the scanner that functions to measure the change in intensity of the reflected light by the alternating light and dark areas within the barcode. The photodiode generates a voltage waveform that is representative of the reflected light and the barcode being read. Decoding circuitry in the scanner interprets the voltage waveform to decode the barcode.
FIG. 1 depicts a laser output window 10 of a laser scanner including a front surface 12 (e.g. external) and rear surface 14 (e.g. internal) arranged in substantially parallel relationship to each other. The material or medium used in window 10 has an associated refractive index “n” and a nominal thickness “t.” When a roughly collimated or focused primary laser beam 11 is transmitted in a first propagation direction from the laser diode through a conventional laser output window 10, the beam 11 strikes the window at an angle of incidence θ and is refracted at an angle of refraction φ after the beam penetrates the window medium. Low-power secondary laser beam reflections typically occur at both the front and rear surfaces 12, 14 of the window. The first of these reflected laser beams 13, which occurs at the interface of the front surface 12 of window 10 where the primary beam 11 leaves the output window medium and re-enters air, travels rearward back through the window and reencounters the rear surface 14 of the output window 10. Some of first reflected laser beam 13 continues to travel rearward back into the scanner leaving window 10 along with a small portion of primary laser beam 11 which is reflected rearward from rear surface 14 as shown in FIG. 5. A very small amount of the first reflected laser beam 13, however, is reflected in an opposite forward direction again forming a second reflected laser beam 13′ traveling in the forward propagation direction of the primary laser beam 11. Most of the second reflected laser beam 13′ leaves front surface 12 of the laser output window 10 and roughly follows along substantially in parallel with the primary beam 11 towards the barcode 16 target. The primary and second reflected laser beams 11, 13′ are separated by a distance measured between points A and B as shown in FIG. 1 and result in an optical path difference (OPD) wherein the distance traversed by beams 11 and 13′ at points A and B are different even though the two beams originated at the same front surface 12 of laser output window 10.
In certain instances, some “flying spot” laser barcode scanners have randomly encountered problems accurately reading the barcode while other scanners of the same design and configuration have not been susceptible to these problems. As further described herein in the Detailed Description section, the barcode reading accuracy problems have been attributed to optical signal noise caused by the primary and second reflected laser beams 11 and 13′, respectively. An improved laser output window is therefore desired that minimizes or eliminates these apparently random scanner barcode reading problems.